Dual port memory apparatus operating a low voltage to maintain low operating current during charging and discharging

ABSTRACT

A semiconductor memory apparatus able to operate at a low voltage and thus preventing an increase of the operating current during charging and discharging. NMOS transistors are connected to the power supply line and bit lines, and the gates thereof are connected to a precharge signal supply line. PMOS transistors are connected to the connection points of the bit lines and sense amplifiers and the supply line of the power supply voltage. The gates thereof are connected to the precharge signal supply line through inverters. Transfer gates are connected to the connection points of the bit lines and the NMOS transistors. The gates thereof are connected to the column switch signal supply line. Only one bit line of the selected column is precharged to the power supply voltage level. The other bit lines are held at the predetermined low potential.

This application is a continuation of application Ser. No. 08/346,179 filed Nov. 22, 1994 now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor memory apparatus for precharging bit lines at a predetermined electric potential to read data therefrom.

2. Description of the Related Art

FIG. 1 is a circuit diagram of an example of the configuration of a static random access memory (SRAM). The drawing shows an example of a dual port SRAM having a single bit line to which precharging is performed. No writing circuit is shown in this drawing.

In FIG. 1, CELL(1,1) to CELL(m,n) represent SRAM cells arranged in a matrix of m lines and n rows, R₋₋ B1 and R₋₋ B2 to R₋₋ B_(n) represent read bit lines, R₋₋ W1 and R₋₋ WB2 to R₋₋ W_(m) represent read word lines, W₋₋ W1 and W₋₋ W2 to W₋₋ W_(m) represent write word lines, NT_(PR1) and NT_(PR2) to NT_(PRn) represent n-channel (N) metal oxide film semiconductor (NMOS) transistors for precharging the bit lines, NTSA_(PR1) represents a NMOS transistor for precharging an input node of a sense amplifier, NT_(SW1) and NT_(SW2) to NT_(SWn) represent NMOS transistors serving as column switches, PU represents a precharge signal supply line, R₋₋ C1 and R₋₋ C2 to R₋₋ Cn represent column switch supply lines, and SA represents a sense amplifier.

In FIG. 1, the SRAM cells CELL (1,1) to CELL (m,n) are TFT load type memory cells. Each, for example, the SRAM CELL (1,1), comprises a flip-flop including a pair of complementary (C) MOS inverters INV_(CL1) and INV_(CL2), inputs and outputs thereof being connected a cross-wise.

The memory nodes, which are outputs of the inverters INV_(CL1) in the SRAM cells CELL (1,1) to CELL (m,n), are connected to the read bit lines R₋₋ B1, R₋₋ B2 to R₋₋ Bn through the word transistors R₋₋ WT. The gates of the respective word transistors R₋₋ WT are connected to the read word lines R₋₋ W1 and R₋₋ W2 to R₋₋ Wm respectively.

The outputs of the respective inverters INV_(CL1) and the outputs of the inverters INV_(CL2) are connected to write bit lines, not shown, through the word transistors W₋₋ WT1 and W WT2, respectively. The gates of the word transistors W₋₋ WT1 and W₋₋ WT2 are connected to the write word lines W₋₋ W1 and W₋₋ W2 to W₋₋ Wm, respectively.

Specifically, the SRAM cells CELL(1,1) and CELL(2,1) to CELL(m,1) are connected to the bit line R₋₋ B1 through the respective word transistors R₋₋ WT, the SRAM cells CELL(1,2) and CELL(2,2) to CELL(m,2) are connected to the bit line R₋₋ B2 through the respective word transistors R WT, and the SRAM cells CELL(1,n) and CELL(2,n) to CELL(m,n) are connected to the bit line R₋₋ Bn through the respective word transistors R₋₋ WT.

Also, the SRAM cells CELL(1,1) and CELL(1,2) to CELL(1,n) are connected to the read word line R₋₋ W1 and the write word line W₋₋ W2, the SRAM cells CELL(2,1) and CELL(2,2) to CELL(2,n) are connected the read word line R₋₋ W2 and the write word line EW2, and the SRAM cells CELL(m,1) and CELL(m,2) to CELL(m,n) are connected to the read word line R₋₋ Wm and the write word line W₋₋ Wm.

The drains of the precharging NMOS transistors NT_(PR1) and NTPR₂ to NT_(PRn) are connected to the power supply lines of the power supply voltage Vdd, and gates of the NMOS transistors NT_(PR1) and NT_(PR2) to NT_(prn) are connected to the common precharge signal supply line PU.

The source of the NMOS transistor NT_(PR1) is connected to the bit line R₋₋ B1, the source of the NMOS transistor NT_(PR2) is connected to the bit line R₋₋ B2, and the source of the NMOS transistor NT_(PRn) is connected to the bit line R₋₋ Bn.

The NMOS transistor NT_(SW1) used as a column switch is provided between a connection point of the bit line R₋₋ B1 and the source of the NMOS transistor NT_(PR1) and a connection point of the bit line R₋₋ B1 and the sense amplifier SA. The gate of the NMOS transistor NT_(SW1) is connected to the column switch signal supply line R₋₋ C1.

The NOS transistor NT_(SW2) used as a column switch is provided between a connection point of the bit line R₋₋ B2 and the source of the NOMS transistor NTPR₂ and a connection point of the bit line R₋₋ B2 and the same amplifier SA. The gate of the NMOS transistor NT_(SW2) is connected to the column switch signal supply line R₋₋ C2.

The NMOS transistor NT_(SWn) used as a column switch is provided between a connection point of the bit line R₋₋ Bn and the source of the NMOS transistor NT_(PRn) and a connection point of the bit line R₋₋ Bn and the sense amplifier SA. The gate of the NMOS transistor NT_(SWn) is connected to the column switch signal supply line R₋₋ Cn.

The drain of the NMOS transistor NTSA_(PR1) for precharging the input node of the sense amplifier SA is connected to the supply line of the power supply voltage vdd, the source of the NMOS transistor NTSA_(PR1) is connected to the node ND_(SA), which is a connection point between the bit lines R₋₋ B1 and R₋₋ B2 to R₋₋ Bn and the sense amplifier SA, and the gate of the NMOS transistor NTSA_(PR1) is connected to the precharge signal supply line PU.

The sense amplifier SA is comprised of inverters INV_(SA1) INV_(SA2) and an NMOS transistor NT_(ST1).

The inverter INV_(SA1) and the inverter INV_(SA2) are connected in series, the input of the inverter INV_(SA1) is connected to the node ND_(SA) which is a connection point between the bit lines R₋₋ B1 and R₋₋ B2 to R₋₋ Bn and the NMOS transistor NTSA_(PR1), and to the source of the NMOS transistor NT_(SA1). The output of the inverter INV_(SA2) functions as the output of the sense amplifier SA, and the output of the inverter INV_(SA2) is connected to the gate of the NMOS transistor ST_(SA1). The drain of the NMOS transistor NT_(SA1) is connected to the supply line of the power supply voltage Vdd.

Next, an explanation will be made of the data read operation of the circuit shown in FIG. 1 with reference to the timing chart in FIG. 2.

First, the read word lines R₋₋ W1 to R₋₋ Wn are set to the low level, and the precharge signal supply line PU is set to the high level.

As a result, the precharging NMOS transistors NT_(PR1) to NT_(PRn) are turned on. The respective bit lines R₋₋ B1 to R₋₋ Bn are precharged at high level. Note that, an actual precharge level is defined as (Vdd-Vth-ΔVth). Here, Vth is the threshold voltage of the transistors, and ΔVth is the change of the threshold due to the substrate bias effect.

Also, since the precharge signal supply line PU is set to high level, the NMOS transistor NTSA_(PR1) is turned on, and the connection point between each of the read bit lines R₋₋ B1 to R₋₋ Bn and the sense amplifier SA, namely, the input node ND_(SA) of the sense amplifier SA, is precharged at a high level.

Next, the level of the precharge signal supply line PU is changed from the high level to the low level, and the read word line R₋₋ Wi selected by the address signal is set to the high level. As a result, the NMOS transistors NT_(PR1) to NT_(PRn) and NTSA_(PR1) are turned OFF.

In response to the data stored in the SRAM cells CELL(i,1) to CELL(i,n) which are connected to the read word line R₋₋ Wi set at a high level, the respective read bit lines R₋₋ B1 to R₋₋ Bn are discharged to a low level or are held at a high level.

One of n rows of the column switch signal supply bias R₋₋ C1 to R₋₋ Cn is set to the high level in accordance with the column address signal. As a result, one column switch NMOS transistor, whose gate is connected to the column switch signal supply line set to the high level, is turned ON, and the signal passing through the column switch NMOS transistor is input to the inverter INV_(SA1) in the sense amplifier SA.

In the above-mentioned apparatus, however, when the power supply voltage Vdd becomes low, the resultant precharge level is lowered, and, as a result, the sense amplifier SA may not amplify the input signal as in normal times.

When the sense amplifier SA is of a type of a sense amplifier in which a gate of an NMOS transistor receives the input signal, for example, a CMOS inverter, the high level input signal applied to the gate must be higher than the threshold voltage Vth of the transistor. That is, to ensure the normal operation of the sense amplifier SA, the following relationship must be satisfied:

    (Vdd-Vth-ΔVth≧Vth)

    ∴(Vdd≧2Vth+ΔVth)

This, however, means that the sense amplifier SA cannot operate normally at the low power supply voltage.

As shown in FIG. 3, there has been proposed a circuit in which the precharging of the bit lines is carried out by p-channel MOS (PMOS) transistors PT_(PR1) to PT_(PRn) and PTSA_(PR1), instead of the NMOS transistors. In the circuit, the column switches are comprised of transfer gates TFG_(SW1) to TFG_(SWn), which are comprised of NMOS transistors and PMOS transistors, with their source and their drains mutually connected, instead of the NMOS transistors. The respective gates of the NMOS transistors N₁ to N_(n) in the transfer gates TFG_(SW1) to TFG_(SWn) are directly connected to the column switch signal supply lines R₋₋ C1 to R₋₋ Cn. The gates of the PMOS transistors P₁ to P_(n) in the transfer gates TFG_(SW1) to TFG_(SWn) are connected to the column switch supply lines R₋₋ C1 to R₋₋ Cn through the inverters INV_(SW1) to INV_(SWn).

In this circuit, the power supply voltage by which the high level signal to the sense amplifier exceeds the threshold voltage Vth is Vdd>Vth. Thus, in this circuit, as compared with NMOS transistors, the sense amplifier can operate at a low level power supply voltage.

But in this case, since the bit line amplitude is increased from (Vdd-Vth-ΔVth) to Vdd, it suffers from the disadvantages of the increase of the operating current of the bit lines by charging and discharging.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a semiconductor memory device operable at a low voltage and thus able to operate with a low power supply by avoiding the increase of the operating current due to charging and discharging.

To achieve the above object, the present invention provides a semiconductor memory apparatus which comprises a plurality of bit lines connected to memory cells, a plurality of n-channel metal insulation film semiconductor (NMIS) transistors connected to a power source and the bit lines, the gates thereof being supplied with a common recharge signal, a p-channel type metal insulation film semiconductor (PMIS) transistor connected to a connection point of the power source line and all bit lines, the gate thereof being supplied with the precharge signal, and a plurality of column switches connected to a connection point of the bit lines and the NMIS transistors and a connection point of all bit lines and the PMIS transistor, wherein only one column switch connected to the bit line of the selected column among the column switches is set to a conductive state.

Also, the present invention provides a semiconductor memory apparatus which comprises a plurality of bit lines, a plurality of memory cells each including first and second transistors connected in series between the bit line and the first power source line, the gate of the first transistor connected to the word line, and the gate of the second transistor connected to a memory node, a PMIS transistor connected to the second power source and all bit lines, the gate thereof being supplied with a precharge signal, and a plurality of column switches connected to connection points of the bit lines and the first transistor and connection points of all bit lines and the PMIS transistor respectively, wherein only one column switch connected to the bit line of the selected column among the column switches is set to a conductive state.

According to the present invention, a high level precharge signal is supplied to each gate of the NMIS transistors, and the column switch connected to the bit line of the selected column is set to the conductive state. As a result, all NMIS transistor are turned ON, and the bit lines are precharged to a high level.

The actual precharge level of the bit lines is (Vdd-Vth-ΔVth). Here, Vth is the threshold voltage of the transistor, and ΔVth is the change of the threshold value due to the substrate bias effect.

Also, the gate of the PMIS transistor is supplied with a signal having a phase inverted from that of the precharge signal. As a result, the PMIS transistor is turned ON.

Consequently, on the terminal sides of the bit lines, for example, the connecting node between each of the bit lines and the sense amplifier is precharged to the power supply voltage level. At this time, as only one column switch in the bit lines of the selected column, among the column switches provided in the bit lines, is made conductive, the level of the selected bit line becomes the power supply voltage level, and the level of other bit lines are held at the level (Vdd-Vth-ΔVth).

Next, the precharge signal is changed from a high level to a low level, and the word line selected by a address signal is set to a high level. As a result, the NMIS and PMIS transistors are turned OFF.

In accordance with the data stored in the memory cells connected to the word line set to a high level, the bit lines are discharged to a low level or are held at a high level.

Further, according to the present invention, all bit lines are not precharged to the predetermined potential at the read operation, so the data stored in the memory cells are not destroy by the provision of the second transistor. Thus, only one bit line of the selected column is precharged.

Namely, a low level precharge signal is applied to the PMIS transistor, and only one column switch connected to the bit line of the selected column is set to the conductive state.

As a result, the PMIS transistor is turned ON, and only one bit line of the selected column is precharged to the second power supply voltage level. At this time, the previous data remains on other bit lines.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and features of the present invention will become more apparent from the following more detailed description of the preferred embodiments with reference to the accompanying drawings, in which:

FIG. 1 is a circuit diagram of the first example of a semiconductor memory apparatus of the related art:

FIG. 2 is a timing chart for explaining the operation of the circuit in FIG. 1;

FIG. 3 is a circuit diagram of a second example of a semiconductor memory apparatus of the related art;

FIG. 4 is a circuit diagram of a first embodiment of a semiconductor memory apparatus according to the present invention;

FIG. 5 is a timing chart for explaining the operation of the circuit in FIG. 4;

FIG. 6 is a circuit diagram of a second embodiment of a semiconductor memory apparatus according to the present invention; and

FIG. 7 is a timing chart for explaining the operation of the circuit in FIG. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will now be explained in detail with reference to the drawings.

FIG. 4 is a circuit diagram of a first embodiment of a semiconductor memory apparatus according to the present invention.

In FIG. 4, CELL(1,1) to CELL(m,n) represent SRAM cells arranged in a matrix of m lines and n rows, R₋₋ B1 and R₋₋ B2 to R₋₋ Bn represent read bit lines, R₋₋ W1 and R₋₋ W2 to R₋₋ Wm represent read word lines, W₋₋ W1 and W₋₋ W2 to W₋₋ Wm represent write word lines, NT_(PR1) and NTPR₂ to NT_(PRn) represent NMOS transistors for precharging the bit lines, PTSA_(PR1) represents a PMOS transistor for precharging an input node of a sense amplifier, INV_(PR1) represents an inverter for precharging, TFG_(SW1) and TFG_(SW2) to TFG_(SWn) represent transfer gates functioning as column switches, PU represents a precharge signal supply line, R₋₋ C1 and R₋₋ C2 to R₋₋ Cn represent column switch supply lines, SA represents a sense amplifier.

The drains of the precharging NMOS transistors NT_(PR1) and NT_(PR2) to NT_(PRn) are connected to the supply lines of the power supply voltage Vdd respectively, and the gates of the NMOS transistors NT_(PR1) and NT_(PR2) to NT_(PRn) are connected to a common precharge signal supply line PU.

The source of the NMOS transistor NT_(PR1) is connected to the read bit line R₋₋ B1, the source of the NMOS transistor NT_(PR2) is connected to the read bit line R₋₋ B2, and the source of NMOS transistor NT_(PR1) is connected to the read bit line R₋₋ Bn.

The drain of the PMOS transistor PTSA_(PR1) precharging the input node of the sense amplifier SA is connected to the supply line of the power supply voltage Vdd. The source thereof is connected to the node ND_(SA), which is a connection point between the read bit lines R₋₋ B1 and R₋₋ B2 to R₋₋ Bn and the sense amplifier SA, and the gate thereof is connected to the output of the inverter INV_(PR1). The input of the inverter INV_(PR1) is connected to the precharge signal supply line PU.

The transfer gate TFG_(SW1) used as a column switch is provided between a connection point of the read bit line R₋₋ B1 and the source of the NMOS transistor NT_(PR1) and the input node ND_(SA) of the sense amplifier SA. The gate of the NMOS transistor N₁ forming the transfer gate TFG_(SW1) is connected to the column switch signal supply line R₋₋ C1, the gate of the PMOS transistor P₁ also forming the transfer gate TFG_(SW1) is connected to the output of the inverter INV_(SW1), and the input of the inverter INV_(SW1) is connected to the column switch signal supply line R₋₋ C1.

The transfer gate TFG_(SW2) used as a column switch is provided between a connection point of the read bit line R₋₋ B2 and the source of the NMOS transistor NT_(PR2) and the input node ND_(SA) of the sense amplifier SA. The gate of the NMOS transistor N₂ forming the transfer gate TFG_(SW2) is connected to the column switch signal supply line R₋₋ C2, the gate of the PMOS transistor P₂ also forming the transfer gate TFG_(SW2) is connected to the output of the inverter INV_(SW2), and the input of the inverter INV_(SW2) is connected to the column switch signal supply line R₋₋ C2.

The transfer gate TFG_(SWn) used as a column switch is provided between a connection point of the read bit line R₋₋ Bn and the source of the NMOS transistor NT_(PRn) and the input node ND_(SA) of the sense amplifier SA. The gate of the NMOS transistor N_(n) forming the transfer gate TFG_(SWn) is connected to the column switch signal supply line R₋₋ Cn, the gate of the PMOS transistor P_(n) also forming the transfer gate TFG_(SWn) is connected to the output of the inverter INV_(SWn), and the input of the inverter INV_(SWn) is connected to the column switch signal supply line R₋₋ Cn.

The sense amplifier SA is comprised of an inverter INV_(SA1) and a PMOS transistor PT_(SA1).

The input of the inverter INV_(SA1) is connected to the node ND_(SA) which is a connection point between the bit lines R₋₋ B1 and R₋₋ B2 to R₋₋ Bn and the drain of the PMOS transistor PT_(SA1). The output of the inverter INV_(SA1) functions as the output of the sense amplifier SA and is connected to the gate of the PMOS transistor PT_(SA1). The source of the PMOS transistor PT_(SA1) is connected to the supply line of the power supply voltage Vdd.

Next, an explanation will be made of the operation when only the bit line R₋₋ B1 is precharged to the power supply voltage Vdd level, with reference to the timing chart in FIG. 5.

First, the read word lines R₋₋ W1 to R₋₋ Wn are set to a low level. The precharge signal supply line PU and the column switch signal supply line R₋₋ C1 are set to a high level. As a result, the NMOS transistors NT_(PR1) to NT_(PRn) are turned ON, the bit lines R₋₋ B1 to R₋₋ Bn are precharged at a high level, the transfer gate TFG_(SW1) is turned ON, and the other transfer gate TFG_(SW2) to TFG_(SWn) are still held at on OFF state. At the time, the actual precharge level of the bit lines R₋₋ B1 to R₋₋ Bn is (VDD-Vth-ΔVth), where Vth is the threshold voltage of the transistor, and ΔVth is the change of threshold voltage due to the substrate bias effect.

As the precharge signal supply line PU is set to a high level, and the gate of the PMOS transistor PTSA_(PR1) is supplied with a low level signal inverted by the inverter IN_(PR1), the PMOS transistor PTSA_(PR1) is turned ON, and the connection point of bit lines R₋₋ B1 to R₋₋ Bn and the sense amplifier SA, namely, the input node ND_(SA) of the sense amplifier SA, is precharged at a high level.

As the precharging transistor is the PMOS transistor, the precharge level of the node ND_(SA) is the Vdd level. At this time, as only the transfer gate TFG_(SW1) provided in the bit line R₋₋ B1, among the transfer gates serving as the column switches in the bit lines R₋₋ B1 to R Bn, is in an ON state, the precharged level of the bit line R₋₋ B1 becomes the Vdd level. The other bit lines R₋₋ B2 to R Bn are held at a level of (Vdd-Vth-ΔVth).

Next, the level of the precharge signal supply line PU is changed from a high level to a low level, and the read word line R₋₋ W1 selected by the address signal is set to a high level. As a result, the NMOS transistors NT_(PR1) to NT_(PRn) and the PMOS transistor PTSA_(PR1) are turned OFF.

In accordance with the data stored in the SRAM cells CELL(i,1) to CELL(i,n), which are connected to the read word line R₋₋ W1 set to the high level, the read bit lines R B1 to R₋₋ Bn are discharged to a low level or are held at a high level.

As explained above, in the present embodiment, the semiconductor memory device (apparatus) is provided with (a) the NMOS transistors NT_(PR1) to NT_(PRn) for precharging the bit lines and connected between the supply lines of the power supply voltage Vdd and the bit lines R₋₋ B1 to R₋₋ Bn, the gates thereof being connected to the common precharge signal supply line PU, (b) the PMOS transistor PTSA_(PR1), connected to the node ND, which is the connection point of the bit R₋₋ B1 to R₋₋ Bn and the sense amplifier SA, the gate thereof being connected to the precharge signal supply line PU through the inverter INV_(PR1), and (c) the transfer gates TFG_(SW1) to TFG_(SWn) provided between the node ND_(SA) and a connection point of the bit lines R₋₋ B1 to R₋₋ Bn and the NMOS transistors NO_(PR1) to NO_(PR2), the gates connected to the column switch signal supply lines R₋₋ C1 to R₋₋ Cn. In the semiconductor memory apparatus, only the precharge signal and the column switch signal supply line of the selected column are set to a high level, only the bit of the selected column is precharged at the power supply voltage Vdd, and the other non-selected bit lines are held at the level (Vdd-Vth-ΔVth) lower than the voltage Vdd. Accordingly, it is possible to operate the sense amplifier at a low voltage and prevent the increase of the operating current due to charging and discharging.

Note that, in the present embodiment, an explanation was made of a semiconductor memory apparatus, of a single read bit line system wherein a single part is used for reading data but the present invention can be applied to multiple port semiconductor memory apparatuses as well.

Further, in the present embodiment, an explanation was made of SRAM cells as memory cells, but the present invention can be applied to other memory cells with bit lines requiring recharging.

FIG. 6 is a circuit diagram of a second embodiment of a semiconductor memory apparatus according to the present invention.

The differences of this embodiment from the first embodiment are that instead of providing the precharging NMOS transistors NT_(PR1) to NT_(PRn), an NMOS transistor NTR for preventing the destruction of data is provided in each of the SRAM cells CELL(1,1) to CELL(m,n), the precharge signal is activated at a low level, and the precharge signal supply line PU₋₋ is directly connected to the gate of the PMOS transistor PTSA_(PR1) for precharging the input node of the sense amplifier SA.

In the concrete configuration of the SRAM cell in the present embodiment, the NMOS transistor NTR is provided between the word transistor R₋₋ WT connected to the corresponding bit line among the bit lines R₋₋ B1 to R₋₋ Bn and the ground, and the gate of the NMOS transistor NTR is connected to the memory node ND_(CL1) of the memory cell.

In the configuration as well, unless all bit lines R B1 to R₋₋ Bn are precharged to the predetermined potential, the data stored in the memory cells are not destroyed by providing the NMOS transistor. Thus, actually, only the bit line of the selected column is precharged.

Next, an explanation will be made of the operation when only the bit line R₋₋ B1 is precharged to the power supply voltage level, with reference to the timing chart in FIG. 7.

First, the read word lines R₋₋ W1 to R₋₋ Wn are set to a low level, only the column switch signal supply line r₋₋ C1 is set to a high level, and the precharge signal supply line PU₋₋ is set to a low level. As a result, the transfer gate TFG_(SW1) is turned ON, the other transfer gates TFG_(SW2) to TFG_(SWn) are held at the OFF state, the PMOS transistor PTSA_(PR1) is turned ON, and the input node ND_(SA) of the sense amplifier SA, which is a connection point of the bit lines R₋₋ B1 to R₋₋ Bn and the sense amplifier SA, is precharged to a high level. As the precharging transistor is the PMOS transistor, the precharge level of the node ND_(SA) is the Vdd level.

At this time, as only the transfer gate TFG_(SW1) provided in the bit line R₋₋ B1 among the transfer gates as the column switches provided in the bit lines R₋₋ B1 to R₋₋ Bn is in the ON state, only the bit line R₋₋ B1 is precharged to the Vdd level. The previous data remain on the other bit lines R₋₋ B2 to R₋₋ Bn.

Next, the level of the precharge signal supply line PU is changed from a low level to a high level, and the read word line R₋₋ W1 selected by the address signal is set to a high level. As a result, the PMOS transistor PYSA_(PR1) is turned OFF.

As explained above, according to the present embodiment, in addition to the effects of the first embodiment, there is the advantage that the charging and discharging are carried out with respect to only the bit line of the selected column, so the power consumption can be further reduced.

As explained above, the semiconductor memory apparatus according to the present invention can operate at a low voltage and does not suffer from an increase of the operating current during charging and discharging. 

What is claimed is:
 1. A semiconductor memory apparatus comprising:a plurality of bit lines connected to a plurality of memory cells; a plurality of first transistors each of which is connected between an ungrounded, power source line and one of said bit lines, wherein the gates of said transistors are supplied with a common precharge signal; a second transistor connected between said power source line and a connection point to which all said bit lines are connected; and a plurality of column switches each one of which is connected to one of said bit lines between one of said first transistors and said connection point, wherein only one of said column switches which is connected to a bit line of a selected column is made conductive at a time; and wherein an operating current of said apparatus is not substantially increased during charging and discharging of said memory cells.
 2. A semiconductor memory apparatus as set forth in claim 1, wherein a gate of said second transistor is supplied with said precharge signal through an inverter so that said precharge signal is inverted as received by said gate of said second transistor.
 3. A semiconductor memory apparatus as set forth in claim 1, wherein a conductivity of said second transistor is different from that of said first transistors.
 4. A semiconductor memory apparatus as set forth in claim 1, further comprising an amplifier connected between said connection point and a data output port.
 5. A semiconductor memory apparats as set forth in claim 4, wherein said amplifier comprises:an inverter connected between said data output port and said connection point; and a third transistor connected between said power source line and said connection point; wherein a gate of said third transistor is connected to said data output port.
 6. A semiconductor memory apparatus as set forth in claim 1, further comprising:a plurality of column switch supply lines; wherein each of said of said column switches comprises:an n-type transistor; a p-type transistor; and an inverter; wherein a gate of said n-type transistor is connected to one of said column switch supply lines, and a gate of said p-type transistor is connect to one of said column switch supply lines through said inverter.
 7. A semiconductor memory apparatus as set forth in claim 6, wherein said plurality of bit lines is equal in number to said plurality of column switch supply lines; and said n-type transistor and said p-type transistor of each column switch are connected to the same column switch supply line.
 8. A semiconductor memory apparatus as set forth in claim 1, further comprising a plurality of read word lines and a plurality of write word lines, wherein one of said read word lines and one of said write word lines are connected to each of said memory cells.
 9. A semiconductor memory apparatus as set forth in claim 1, wherein said precharge signal is high during a precharging period such that said first transistors are made conductive.
 10. A semiconductor memory apparatus as set forth in claim 2, wherein said precharge signal is high during a precharging period such that said first transistors are made conductive; and further wherein, after said precharging period, said precharge signal is made low such that said second transistor is made non-conductive.
 11. A semiconductor memory apparatus as set forth in claim 8, wherein said precharge signal and said plurality of read word lines are low prior to a precharging period.
 12. A semiconductor memory apparatus as set forth in claim 1, further comprising:a plurality of column switch supply lines each of which is connected to one of said column switches; wherein said precharge signal and one of said column switch supply lines for a selected column are high during a precharging period. 